Experimental hookworm (Necator americanus) infections were induced in normal volunteers with a goal toward defining progressive changes in immunologic responses (especially IgE) to helminth infection. It was found that a small infecting dose of 50 filariform larvae was capable of producing significant transient gastrointestinal morbidity and marked blood eosinophilia but induced other T-cell and B-cell dependent immune responses that were of such low magnitude as to be difficult to evaluate extensively. Clear changes in eosinophil structural and functional profiles could be documented during the course of the infections. The proportion of 'hypodense' eosinophils increased two- to three-fold during infection while the capabilities of generating superoxide and of migrating to a chemotactic stimulus also increased significantly. Such changes were limited to eosinophils and were not seen in the neutrophil granulocyte population.